In general, when ethylene-propylene-diene-monomers (hereinafter referred to as EPDM) are prepared using a vanadium catalyst, catalyst residues remaining in the polymer are removed using a NaOH aqueous solution and high-pressure steam in order to remove Cl contained in the catalyst from a polymer composition. However, this method is problematic in that a laborious procedure of removing the catalyst in the polymer is required so as to make the process complex.
Further, the process of preparing EPDM using the vanadium catalyst requires much energy, because high-pressure steam is directly fed to the polymer solution in order to recover unreacted monomers, comonomers, and the solvent. The unreacted monomers, comonomers, and solvent removed from the polymer composition are recycled to a reactor after water is removed therefrom by an additional purification process, leading to high purification costs. For example, to prepare 10 tons of EPDM polymers per hour, a total amount of monomers, comonomers, and hexane excluding the EPDM polymer to be fed to the inlet of the reactor is 50 to 100 tons, and to separate them from the polymer and to recycle them to the reactor, catalyst poisons such as water and a catalyst killing agent injected during the process must be eliminated. However, the conventional method has an inefficiency problem of requiring excessive energy during the process.
Further, in the conventional method of preparing EPDM using the vanadium catalytic composition, the solvent and unreacted monomers are steam-stripped in order to recover the polymer. Korean Patent No. 10-0226175 describes a method of removing catalyst residues of the vanadium catalytic composition using water or an aqueous alkaline solution. U.S. Pat. No. 3,590,026 also describes a method of removing the solvent and unreacted monomers in a two-stage steam stripping system. However, removal of the solvent and unreacted monomers from the polymer solution by steam-stripping is also disadvantageous in that it requires energy consumption that is twice or higher than that of the present invention described below, resulting in excessive costs.
To overcome this problem, Korean Patent No. 10-0496101 discloses a method of recovering the polymer by a two-stage anhydrous solvent recovery process upon preparation of EPDM using a metallocene catalytic composition. This method has an advantage of using less energy, but a disadvantage of poor fluidity of the polymer solution when it is used to prepare EPDM polymers having high viscosity and high molecular weight. Further, when a devolatilization extruder is used, a polymer having high density is produced and thus it is necessary to blend with oil, carbon black, or an accelerator for compounding. However, there is a disadvantage of its poor mixing capacity. That is, EPDM having a high molecular weight cannot be prepared by the above method, and after purification, excessively high shear is applied to the extruder, resulting in unfavorable production.
Accordingly, there is a continuous demand for a method of preparing an elastic EPDM copolymer, which is used to prepare EPDM having a high molecular weight and to improve productivity and energy efficiency.